1. Field of the Invention
The present invention relates to a switching control circuit preferably in the form of an IC and preferably for use in a switching power supply apparatus, and a switching power supply apparatus including the switching control circuit.
2. Description of the Related Art
In switching power supply apparatuses, a switching control IC includes a circuit for providing various functions such as an output control operation, a startup operation, an overcurrent protection operation, an overvoltage protection operation, a standby operation, and a power factor improvement operation. With the increase in the number of functions, the functionality of the switching control IC is increased.
In order to set the functions of a switching control IC so that they meet application operational specifications, a plurality of terminals used to interface with external circuits are needed for the respective functions. Accordingly, the increase in the number of functions increases the number of terminals. The increase in the number of terminals results in an increase in the package size of the switching control IC. This leads to an increase in the unit cost of the IC.
In a case where the number of terminals of a switching control IC is limited, the number of available functions is also limited. It is therefore necessary to prepare various ICs corresponding to specific functions and selectively use the ICs in accordance with specifications and application. In this case, the variety of ICs is increased. Not only an IC manufacturing process but also IC management becomes complicated. As a result, the unit cost of ICs is increased.
In general, ICs inevitably increase in size to meet the increasing demand for multifunctionality. However, the number of functions required for small-sized switching control circuits have recently been increased. Under the present conditions, the mass production of a small variety of ICs is an effective way to reduce the unit cost of semiconductors such as ICs. In addition, the reduction in the number of terminals of an IC, which leads to the reduction in the size of an IC package, can reduce the unit cost of the IC.
Japanese Unexamined Patent Application Publication No. 2007-73954 discloses a technology for reducing the number of terminals of a switching control IC.
FIG. 1 is a circuit diagram of a switching power supply apparatus disclosed in Japanese Unexamined Patent Application Publication No. 2007-73954. Referring to FIG. 1, a switching power supply apparatus 101 includes a transformer 105 having a primary winding 127 and a secondary winding 129, a rectifying and smoothing circuit including a diode 117 and a capacitor 119, a feedback circuit including a Zener diode 121, a photocoupler 113, and a resistor 123, and an integrated circuit 103.
The integrated circuit 103 is connected to the primary winding 127. The integrated circuit 103 is a switching regulator IC including an internal switch coupled between a drain (D) terminal and a source (S) terminal of the integrated circuit 103.
During operation, the internal switch in the integrated circuit 103 adjusts energy transmission from an input 107 to an output 109 through the transformer 105. A feedback signal is input from the feedback circuit to the integrated circuit 103.
A multifunctional capacitor 111 is connected to a bypass terminal BP of the integrated circuit 103. The multifunctional capacitor 111 is used to provide a power supply decoupling function for the integrated circuit 103 during normal operation. The internal circuit in the integrated circuit 103 receives power or a bias current from the multifunctional capacitor 111 and operates during normal operation while adjusting the output 109.
The multifunctional capacitor 111 is used to select a parameter and/or a mode of the integrated circuit 103 during an initialization period of the integrated circuit 103. During the initialization period, the parameter and/or the mode of the integrated circuit 103 is selected.
As described previously, the functions of a switching power supply apparatus include an overcurrent protection function. Methods of implementing the overcurrent protection function include a method of limiting the maximum ON pulse width of a switching element and a method of limiting a peak current. In these methods, a unit for setting a limit value is needed.
However, in a case where switching control ICs for the overcurrent protection methods are independently designed and manufactured, the variety of switching control ICs is increased. Not only an IC manufacturing process but also an IC management becomes complicated. As a result, the unit cost of ICs is increased. In a case where switching control ICs individually have the overcurrent protection function, a terminal for the selection of the overcurrent protection function is needed in each of the switching control ICs. The number of terminals of the switching control IC is increased. This inhibits the miniaturization of the switching control IC and increases the cost of the switching control IC. That is, the size and unit cost of the switching control IC are increased.